Re: Why doesn't Postgres apply limit on groups when retrieving N results per group using WHERE IN + ORDER BY

This is related to a question I asked on dbs.stackexchange.com: https://dba.stackexchange.com/questions/335501/why-doesnt-postgres-apply-limit-on-groups-when-retrieving-n-results-per-group

But to reiterate - I have a query like this:

SELECT "orders".* 

FROM "orders" 

WHERE (user_id IN ?, ?, ?) 

ORDER BY "orders"."created_at" LIMIT 50

So my question is twofold: 

- why doesn't Postgres use the composite index, and then retrieve only the minimum necessary amount of rows (50 per user) using the query I posted above?